Production of the antimicrobial compound tetrabromopyrrole and the Pseudomonas quinolone system precursor, 2-heptyl-4-quinolone, by a novel marine species Pseudoalteromonas galatheae sp. nov

Abstract

Novel antimicrobials are urgently needed due to the rapid spread of antibiotic resistant bacteria. In a genome-wide analysis of Pseudoalteromonas strains, one strain (S4498) was noticed due to its potent antibiotic activity. It did not produce the yellow antimicrobial pigment bromoalterochromide, which was produced by several related type strains with which it shared less than 95% average nucleotide identity. Also, it produced a sweet-smelling volatile not observed from other strains. Mining the genome of strain S4498 using the secondary metabolite prediction tool antiSMASH led to eight biosynthetic gene clusters with no homology to known compounds, and synteny analyses revealed that the yellow pigment bromoalterochromide was likely lost during evolution. Metabolome profiling of strain S4498 using HPLC-HRMS analyses revealed marked differences to the type strains. In particular, a series of quinolones known as pseudanes were identified and verified by NMR. The characteristic odor of the strain was linked to the pseudanes. The highly halogenated compound tetrabromopyrrole was detected as the major antibacterial component by bioassay-guided fractionation. Taken together, the polyphasic analysis demonstrates that strain S4498 belongs to a novel species within the genus Pseudoalteromonas, and we propose the name Pseudoalteromonas galatheae sp. nov. (type strain S4498^T = NCIMB 15250^T = LMG 31599^T).

Figure 1

Phylogenomic tree of Pseudoalteromonas species based on genome distances as provided by the TYGS platform. Values are pseudo-bootstraps with average branch support of 59.6%. The tree is rooted at the midpoint.

Figure 2

Phylogenetic tree of all alleles of 16S rRNA genes in Pseudoalteromonas genomes considered ‘complete’ in NCBI. Nodes and label colors correspond to individual genomes. Inner color ring and outer ring denotes species and type strain status (black = type strain, grey = no type strain), respectively. Labels have been compressed to avoid overlapping. P. atlantica T6c^T has been used as outgroup.

Figure 3

Synteny comparisons of genes surrounding (a) the alterochromide cluster in P. piscicida NCIMB 1142^T compared to P. galatheae S4498^T, (b) the pseudane cluster in strain S4498^T compared to P. piscicida NCIMB 1142^T and (c) the tetrabromopyrrole cluster in P. galatheae S4498^T and P. peptidolytica DSM 14001^T as compared to P. piscicida NCIMB 1142^T. Biosynthetic gene clusters are marked with red and differently grey-shaded bars display homology between regions.

Figure 4

Structures of the major identified secondary metabolites detected from P. galatheae S4498^T. (a) Pseudanes III–IX. The structures of pseudane V and pseudane VII were confirmed by NMR spectroscopy, and the identities of the remaining pseudanes were determined by similarities using their MS/MS fragmentation patterns. (b) Structure of tetrabromopyrrole.